Color corrected projection lens

ABSTRACT

A projection lens in which a lens unit which may comprise one or two elements is closely coupled at the object end of the lens to a cathode ray tube. At the image end is a lens unit of overall weak optical power which comprises from the image end a first positive element followed by a closely spaced negative element of substantially higher dispersion. Intermediate the object side lens unit and the image side lens unit is a lens unit of substantial optical power supplying substantially all of the positive optical power of the lens. Closely spaced to this power lens unit on the object side is a negative lens element of high dispersion. This negative lens element may be considered to be part of the power lens unit. The negative lens elements have weak optical power at the axis of the lens but are of generally increasing negative optical power from the optical axis to the clear apertures of the negative lens elements to contribute to proper color correction of the lens, particularly axial chromatic aberration adjacent the full aperture. The intermediate positive power lens unit may comprise one or two positive elements depending upon the application and the manner in which the power is to be distributed to minimize aberrations. A lens embodying the invention is capable of maintaining a high image quality over a large range of magnifications by moving the first and second lens units from the image end in predetermined differential relationship.

FIELD OF THE INVENTION

This invention relates to projection lenses for cathode ray tubes and,more particularly, relates to such lenses which are color corrected.

BACKGROUND OF THE INVENTION

In projection television systems it is a common practice to utilizethree cathode ray tubes (CRT) of different colors, namely, red, blue andgreen. The images formed on each of these CRT's is projected onto ascreen where the three separate images are combined to form a full colorpicture. Lenses used in this application can be generally characterizedby a construction which includes a strong negative element in thevicinity of the CRT, a strong positive power component at some distanceaway from said negative element, and a weaker component furthest awayfrom the CRT. Examples of such lenses are shown in U.S. Pat. Nos.4,300,817; 4,348,081 and 4,526,442.

In practice, the phosphors used in CRT's do not emit strictlymonochromatic light. The emission spectrum of the green CRT, inparticular, has significant side bands in red and blue parts of thespectrum. If projection lenses are not corrected for color, thischromatic spread will result in a lower overall image contrast and insometimes visible color fringing. Therefore, to achieve the imagequality requirements of high end projectors, including data display,intermediate definition and high definition television (IDTV and HDTV)applications, it is desirable to provide some degree of correction forchromatic aberrations in the lens.

Examples of partially color corrected lenses are disclosed in U.S. Pat.Nos. 4,530,575; 4,733,953, 4,758,074; 4,778,264 and 4,815,831. Fullycolor corrected lenses for data display and high definition TVapplications are shown in U.S. Pat. Nos. 4,767,199 and 4,792,218.

The requirement for partial or total color correction always complicatesan optical design problem. To achieve some degree of control ofchromatic aberrations a combination of elements of various dispersionsand powers must be used. That, in turn, makes it more difficult tocorrect for monochromatic aberrations like spherical, coma, astigmatism,etc. In the end, a color corrected lens become more complex, with alarger number of elements, and is more expensive to manufacture than thelens with only monochromatic aberration correction. An increased numberof elements also leads to higher losses in transmission and contrast.And, since the individual elements in these lenses are quite large insize, additional increases in weight of these lenses become significant.

Additionally, it is often desirable that lenses be capable of highperformance levels over a significant range of magnifications. Ideally,in front projection application the same lens might be used over a rangeof magnifications from 10× to 60× without any degradation in imagequality. This further complicates the optical design.

Focusing very fast f/1.0 lenses covering a wide field of view has alwaysbeen difficult. It is desirable to be able to use the same lens. Forexample, the same front projector should be easily adaptable to varioussize auditoriums by simply refocusing the projection lenses without anyloss in image quality, or the lens used on one model of the rearprojector should be also usable on a wide variety of other models andsizes of rear projection TV sets when appropriately refocused.

The most common way of focusing projection TV lenses usually involvesmoving the whole lens along the optical axis. When the rear strongnegative element is liquid coupled to the CRT, only the front portion ofthe lens is moved. This focusing technique works sufficiently well overa limited range of magnifications. Variations of spherical aberrationsand astigmatism usually prevent the range to be extended substantially.When conjugates are changed significantly the lens must be partially orcompletely redesigned.

Accordingly, the present invention provides new and improved colorcorrected projection lenses for CRT's of high definition whilemaintaining a wide field of view and a large relative aperture. Theinvention also provides a CRT projection lens that is capable ofmaintaining a high level of image quality over a wide range ofmagnifications. The invention also provides a lens which achieves a goodimage and a high degree of stability of aberrations correction withshift of conjugates in a relatively simple configuration.

SUMMARY OF THE INVENTION

Briefly stated, the invention in one form thereof comprises a projectionlens which includes from the long conjugate a first unit of twoelements, one positive and one negative, having a weak combined power, asecond lens unit providing most of the positive power of the lens andincluding at least one positive element followed by a negative element,and a third unit which includes a strong negative element in thevicinity of the CRT. Each of the negative elements in the first and thesecond units have at least one aspherical surface.

The longitudinal chromatic aberration or axial color is corrected by anappropriate combination of positive and negative elements with differentdispersion characteristics. The better the correction, the stronger thepowers of a positive and a negative elements must be, or the dispersiondifference between these elements must be increased. By using twonegative elements instead of one the power of each of these negativeelements can be kept weak, with each of the first and the second unitspartially correcting for axial color. Any transverse blur in the imageplane caused by the axial color is proportional to the square of theheight of the axial marginal ray. The size of this aberration at smalleraperture heights is not significant enough to effect the performance ofthe lens. Only when the aperture becomes larger especially for f/1.0lenses, does this aberration become important to correct. Therefore,aspherical surfaces can be used to shape the negative elements in such away so that the correction of axial color is accomplished mainly foroutlying portions of the aperture, that is, in the areas closer to themaximum clear apertures of the elements of the first and the secondunits of the lens. Consequently, the refractive power of these negativeelements, while relatively weak on axis, gradually increases towards theouter portion of the elements. If the surfaces of these elements werespherical with corresponding base radii, the refractive power of theelements would not change across the diameter, and the correction ofcolor would only be partial, and not sufficient to achieve a highperformance level. If, however, the radii of these elements were madeshort enough to obtain an appropriate correction of chromaticaberrations, then additional elements would be needed to obtain anadequate correction of monochromatic aberrations and, also, satisfy thebasic physical specifications of the lens such as focal length, field ofview and relative aperture.

Having negative elements of relatively weak power also helps to keep thevariation of thickness across the diameter of the element reasonablysmall, thus allowing for economical manufacture of these elements bymolding them from out of an appropriate plastic material.

When the field of view of the lens is increased and the opticalperformance requirements are raised to higher levels, it is helpful toadd another element between the second and third lens units. Placing anelement with at least one aspherical surface in that location allows forbetter correction of sagittal oblique spherical aberrations and otherresidual offaxis aberrations.

A new focusing method involves moving the first and the second lensunits along the optical axis in a predetermined fashion relative to eachother. The third lens unit and a corrector, if any, remain fixedrelative to the CRT. The motion of the first two units can be controlledby a cam or a double helocoid to achieve a simultaneous change inposition of these two units. By focusing a lens in this fashion a verystable high quality optical performance can be achieved with a minimumnumber of elements.

In a preferred embodiment the strongest positive elements in the secondlens unit are made out of glass to achieve some thermal stability, asdescribed in the prior art. The rest of the elements are plastic. Thisallows for the most economical manufacturing of the aspherical elements.Because some of the plastic elements have positive power and some havenegative power, and their thermo optic coefficients describing thechange in the refractive index as a function of temperature are similar,it is also possible to improve the thermal stability of the lens evenfurther by choosing appropriate powers for these elements.

An object of this invention is to provide a new an improved colorcorrected lens which has a fast relative aperture, covers a wide fieldof view and is simple and inexpensive to manufacture in largequantities.

Another object of this invention is to provide a novel focusing methodfor a projection TV lens which allows for the lens to achieve a greatstability of image quality over a large range of magnifications.

The features of the invention which are believed to be novel areparticularly pointed out and distinctly claimed in the concludingportion of this specification. The invention, however, together withfurther objects and advantages thereof may best be appreciated byreference to the following detailed description taken in conjunctionwith the drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1, 2, 3 and 4 are schematic side elevations of lenses embodyingthe invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

A first lens embodying the invention as illustrated in FIG. 1 comprisesa first or front lens unit G1 comprised of elements L1 and L2. Lenselement L1 is positive and is of a low dispersion material, similar tocrown glass, such as acrylic and has at least one aspheric surface.Element L1 in lens unit G1 is closely followed by a negative lenselement L2 having high dispersion characteristic of flint glass. Theoptical power at the axis of lens element L2 is very low as willhereinafter be made apparent, lens element L2 has at least one asphericsurface which changes in power towards the clear aperture thereof toprovide correction for axial chromatic aberration. Lens element L1 alsohas an aspheric surface which changes in power towards the clearaperture thereof. The elements L1 and L2 cooperate in powers anddispersion in the vicinities of the clear apertures thereof tocontribute to correction of axial chromatic aberration.

A second or another lens unit G2 comprises the major power element L3 ofthe lens of a material similar to crown type glass followed by anegative lens element L4 of high dispersion which is closely spacedthereto and in which the axial marginal rays AM intersect the lens at aheight very close to the clear aperture to provide further correctionfor axial chromatic aberration.

A third or rear lens unit G3 comprises a lens element L5 and a housingmember identified by the reference numeral L6 which contains a liquidcoolant which is of a compatible index of refraction to lens element L5.The element L6 not only provides negative optical power to contribute tocorrection of field curvature but provides a coolant for the cathode raytube C.

It will be noted that the axial marginal rays as denoted by thereference numeral AM intersect the elements L2 and L4 very close to theclear aperture thereof to provide correction for axial chromaticaberration.

In the lenses described the elements L2 and L4 (FIGS. 1, 2 and 4) andelements L2 and L5 (FIG. 3) are configured in the vicinity of the clearaperture to contribute to correction of axial color by the provision ofat least one aspheric surface which changes the power of the elements inthe vicinity of the clear apertures thereof with respect to the paraxialpower.

A lens as shown in FIG. 1 is substantially described in Table I.

A lens embodying the invention, as shown in FIG. 2, is somewhat similarto the lens of FIG. 1 but it includes a corrector lens unit CT betweenthe second lens unit G2 and lens unit G3. The lens shown in FIG. 2comprises a lens unit G1 having a positive lens L1 and a negative lensL2 of much higher dispersion as measured by its Abbe number. Lens unitG2 comprises a optically strong unit L3 followed by a negative unit L4having a very high dispersion providing color correction for axialchromatic aberrations adjacent the clear aperture thereof. Lens G2 isfollowed by a strongly concave negative lens unit G3 which compriseswhat may be considered a shell element L6 followed by an element whichincludes a liquid coupler L7 of appropriate refractive index betweenelement L6 and a cathode ray tube. The cathode ray tube is curved on thephosphor side thereof to provide some optical power. In this regardsince the interior surface of the CRT contributes optical power, it isincluded in the prescription in Table II for the lens shown in FIG. 2.

In the lens embodiments of FIGS. 1 and 2, the lens element L4 may beconsidered part of lens unit G2 inasmuch as in contributing tocorrection of axial chromatic color aberration it cooperates withelement L3. It will be noted that the axial marginal rays AM in theembodiment shown in FIG. 2 strike the element L4 closely adjacent theclear aperture thereof where it is of strongest power and together withthe preceding positive element contributes to correction of axialchromatic aberration. The axial marginal rays as shown in FIG. 2 strikethe corrector element CT at slightly less than the clear aperturethereof so as to help to contribute to aperture dependent aberrationsbut leave some room at the outer edges thereof to contribute tocorrection of off axis aberrations such as sagittal oblique sphericalaberrations.

A lens as shown in FIG. 2 is substantially described in Table II.

The lens of FIG. 3 is primarily designed for front projection and has alonger equivalent focal length as set forth in Table III. The lens ofFIG. 3 comprises a first lens unit G1 which comprises a positive elementfollowed by a closely spaced negative element of very high dispersion tocontribute correction of axial chromatic aberration particularly at theouter edges thereof. The second lens unit G2 comprises two positiveelements L3 and L4 followed by a negative lens unit L5 which alsoprovides at its outer edges thereof in conjunction with element L4correction for axial chromatic aberration. Element L5 has at least oneaspheric surface, and the absolute power of the lens increases from theaxis thereof toward the clear aperture due to this aspheric surface.

The third lens unit G3 comprises an element L7 which is closely coupledto a cathode ray tube C. In this embodiment there is an X-Ray absorbingplate P and in between a housing H which holds a cooling liquid betweenthe cathode ray tube C and the X-Ray absorbing plate P.

Additionally, a corrector lens unit CT having at least one asphericsurface is positioned between the second lens unit G2 the third lensunit G3.

This corrector lens unit aids in correction of aperture dependentaberrations towards the central area thereof while providing correctionfor off axis aberration at the outer edges thereof.

The embodiment of FIG. 3 may be considered to be air coupled to the CRTwhile the embodiments of FIGS. 1 and 2 are optically coupled to the CRTin view of the liquid containing housing units designated by the lens L6in FIG. 1 and L7 in FIG. 2.

A lens as shown in FIG. 3 is substantially described in Table III.

Lenses as shown in FIG. 4 have a similar configuration to the lens ofFIG. 2, but are adapted for a large range of magnifications as wellhereinafter be described.

Two lenses as shown in FIG. 4 are substantially described in Tables IVand V.

In the lens of FIG. 1, at least one surface of element L1 as well as thedifference in dispersion of the cooperating positive element isconfigured to cooperate with element L2 in the vicinity of the clearaperture to contribute to correction of axial chromatic aberration. Thisis also true in all other examples.

Axial longitudinal chromatic aberration adjacent the clear aperture ofthe lens elements is corrected by the combination of the positive andnegative elements of the lens units G1 and also the positive andnegative elements of the lens unit G2.

It will depend upon the decision of the TV manufacturer as to whether itwishes to have an optically coupled lens as shown in FIGS. 1, 2 and 4 oran air coupled lens as shown in FIG. 3.

In the disclosed embodiment of the invention, lens elements haveaspheric surfaces defined by the equation ##EQU1## where x is thesurface sag at a semi-aperture distance y from the axis A for the lens,C is the curvature of the lens surface at the optical axis, K is a conicconstant, and D, E, F, G, H and I are aspheric coefficients ofcorrespondingly fourth through fourteenth order in even expotentialpowers.

In the following tables, the lens elements are identified from the imageend to object end by the reference L followed by successive Arabicnumerals. Lens surfaces are identified by the reference S followed bysuccessive Arabic numerals from the image to the object end. Positiveradii are struck from the right and negative radii from the left. Theindex of refraction of each element is given under the heading Ne. Thedispersion of each lens element as measured by its Abbe number is givenby Ve.

The index of refraction given by the symbol Ne represents the greenmercury line and the dispersion Ve is also to the green mercury line.Other parameters of disclosed lenses embodying the invention are alsoset forth in Tables I-V, and the following tables.

                                      TABLE I                                     __________________________________________________________________________            SURFACE RADIUS                                                                           AXIAL DISTANCE                                             LENS    (mm)       BETWEEN SURFACES (mm)                                                                          N.sub.e                                                                           V.sub.e                               __________________________________________________________________________        S1  79.7613                                                               L1                 20.0000          1.49354                                                                           57.34                                     S2  -669.9136  4.6902                                                         S3  -116.8472                                                             L2                 7.0000           1.59495                                                                           30.70                                     S4  1056.4020                                                                                23.6442                                                        S5  70.9138                                                               L3                 28.0000          1.64128                                                                           55.19                                     S6  -124.5127                                                                                .5000                                                          S7  -105.1656                                                             L4                 8.2300           1.59495                                                                           30.70                                     S8  -269.9804                                                                                55.3717                                                        S9  -59.4288                                                              L5                 3.7500           1.49354                                                                           57.34                                      S10                                                                              -58.0000                                                              L6                 8.0000           1.42900                                                                           50.00                                     S11 plano                                                                 L7                 5.7500           1.53700                                                                           50.00                                     S12 plano      -.0002                                                     __________________________________________________________________________    ASPHERICAL SURFACE DATA:                                                      S D      E      F      G      H      I                                        __________________________________________________________________________    S1                                                                              -.42555E-06                                                                          -.49543E-09                                                                           .26480E-12                                                                          -.20345E-15                                                                           .61635E-19                                                                          -.12486E-22                              S2                                                                              -.52887E-06                                                                          -.23095E-09                                                                           .60365E-13                                                                           .66225E-16                                                                          -.54544E-19                                                                           .11586E-22                              S4                                                                              -.22688E-07                                                                           .18679E-10                                                                          -.52303E-13                                                                          -.20711E-16                                                                           .90800E-20                                                                          -.15468E-23                              S7                                                                              -.20618E-06                                                                           .78774E-09                                                                          -.42344E-12                                                                           .11201E-15                                                                          -.24723E-19                                                                           .39342E-23                              S8                                                                               .77243E-06                                                                           .68520E-09                                                                          -.21054E-13                                                                          -.15621E-15                                                                           .54388E-19                                                                          -.70704E-23                              S9                                                                              -.19224E-05                                                                           .10054E-09                                                                          -.51595E-13                                                                           .70329E-16                                                                           .51214E-19                                                                          -.42912E-22                              __________________________________________________________________________     EFL = 99.0 mm                                                                 f/NO = 1.10 as infinity                                                  

                                      TABLE II                                    __________________________________________________________________________            SURFACE RADIUS                                                                           AXIAL DISTANCE                                             LENS    (mm)       BETWEEN SURFACES (mm)                                                                          N.sub.e                                                                            V.sub.e                              __________________________________________________________________________        S1  74.2416                                                               L1                 17.5000          1.49354                                                                            57.34                                    S2  1890.7290                                                                                6.3993                                                         S3  -353.7105                                                             L2                 6.0000           1.59495                                                                            30.70                                    S4  168.5545                                                                                 7.9750                                                         S5  76.7012                                                               L3                 29.5000          1.59142                                                                            61.03                                    S6  -102.6833                                                                                .4500                                                          S7  -140.5109                                                             L4                 6.0000           1.59495                                                                            30.70                                    S8  2405.5770                                                                                1.8204                                                         S9  3614.1540                                                             L5                 13.0000          1.49354                                                                            57.34                                    S10 -107.5106                                                                                Z1                                                             S11 -54.2507                                                              L6                 5.0000           1.49354                                                                            57.34                                L7                 5.0000           1.43300                                                                            50.00                                    S13 plano                                                                 L8                 13.1000          1.56600                                                                            50.00                                    S14 -350.0000                                                                                -.0007                                                     __________________________________________________________________________    ASPHERICAL SURFACE DATA:                                                      S  D      E      F      G      H      I                                       __________________________________________________________________________    S1 -.30278E-06                                                                          -.39696E-09                                                                           .65578E-12                                                                          -.54355E-15                                                                           .22972E-18                                                                           -45485E-22                             S2  .50238E-06                                                                           .29645E-09                                                                          -.19419E-12                                                                           .15207E-15                                                                          -.14647E-18                                                                           .35404E-22                             S3  .33721E-06                                                                          -.11255E-09                                                                          -.67345E-13                                                                          -.23116E-16                                                                          -.57597E-20                                                                           .59609E-23                             S4  .45051E-06                                                                          -.39788E-11                                                                           .15672E-12                                                                           .44925E-16                                                                           .36143E-19                                                                          -.11782E-22                             S7 -.17777E-06                                                                           .14069E-08                                                                          -.77347E-12                                                                           .29025E-15                                                                          -.69186E-19                                                                           .35744E-23                             S8  .13689E-05                                                                           .13175E-08                                                                          -.50402E-12                                                                          -.19391E-15                                                                           .35271E-18                                                                          -.53285E-22                             S9  .16754E-05                                                                          -.39851E-09                                                                           .28977E-12                                                                          -.29520E-15                                                                           .20386E-18                                                                           .44467E-22                             S10                                                                               .33475E-06                                                                          -.16086E-09                                                                           .11848E-12                                                                          -.60788E-16                                                                          -.66995E-19                                                                           .82337E-22                             S11                                                                               .61205E-05                                                                           .53626E-08                                                                          -.35544E-11                                                                           .63970E-15                                                                           .29820E-18                                                                           -10191E-21                             __________________________________________________________________________     EFL = 89.4 mm                                                                 f/NO = 1.0 at infinity                                                   

                                      TABLE III                                   __________________________________________________________________________            SURFACE RADIUS                                                                           AXIAL DISTANCE                                             LENS    (mm)       BETWEEN SURFACES (mm)                                                                          N.sub.e                                                                            V.sub.e                              __________________________________________________________________________        S1  118.0413                                                              L1                 19.0000          1.49354                                                                            57.34                                    S2  279.5864                                                                                 20.1020                                                        S3  -270.8739                                                             L2                 9.0000           1.59495                                                                            30.70                                    S4  270.8739                                                                                 14.9593                                                        S5  171.0173                                                              L3                 40.5796          1.51872                                                                            64.02                                    S6  -171.0173                                                                                .2000                                                          S7  83.7713                                                               L4                 35.6330          1.51872                                                                            64.02                                    S8  -1328.3410                                                                               9.9750                                                         S9  2057.5210                                                             L5                 8.0000           1.59495                                                                            30.70                                    S10 85.7104                                                                                  Z1                                                             S11 257.6417                                                              L6                 19.0000          1.49354                                                                            57.34                                    S12 -178.9754                                                                                25.2767                                                        S13 -69.6645                                                              L7                 5.0000           1.49354                                                                            57.34                                    S14 466.9425                                                                                 Z2                                                             S15 plano                                                                 L8                 6.5000           1.53700                                                                            50.00                                    S16 plano                                                                 L9                 4.8100           1.42000                                                                            50.00                                L10                5.7500           1.57200                                                                            50.00                                    S18 plano                                                                                    .0024                                                      __________________________________________________________________________    ASPHERICAL SURFACE DATA:                                                      S  D      E      F      G      H      I                                       __________________________________________________________________________    S1 -.11621E-06                                                                          -.49544E-10                                                                           .18681E-13                                                                          -.67047E-17                                                                           .11303E-20                                                                          -.69308E-25                             S2  .38060E-08                                                                          -.62373E-11                                                                          -.25648E-14                                                                           .27436E-17                                                                          -.54489E-21                                                                           .69514E-25                             S3 -.32858E-07                                                                          -.47960E-11                                                                          -.16998E-15                                                                           .21168E-18                                                                           .19367E-22                                                                          -.78372E-26                             S4  .32858E-07                                                                           .47960E-11                                                                           .16998E-15                                                                          -.21168E-18                                                                          -.19367E-22                                                                           .78372E-26                             S9 -.18988E-06                                                                           .79851E-10                                                                          -.28921E-13                                                                           .54158E-17                                                                          -.50610E-21                                                                          -72986E-27                              S10                                                                               .11973E-06                                                                           .15072E-09                                                                          -.23489E-13                                                                           .81292E-17                                                                           .23668E-20                                                                          -.55905E-24                             S11                                                                                51476E-07                                                                           .13917E-10                                                                          -.21354E-13                                                                           .96526E-18                                                                            24004E-20                                                                          -.85472E-24                             S12                                                                              .30763E-06                                                                           -.82701E-10                                                                          -.79401E-14                                                                           .40372E-17                                                                          -.55063E-22                                                                          -.11513E-23                             S13                                                                               .16025E-06                                                                          -.33837E-09                                                                           .82317E-13                                                                           .33406E-16                                                                          -.16682E-19                                                                           .23473E-24                             S14                                                                              -.47493E-06                                                                          -.77318E-10                                                                           .29802E-13                                                                          -.57967E-17                                                                           .25949E-21                                                                          -.40005E-25                             __________________________________________________________________________     EFL = 139.4 mm                                                                f/No = 1.0 at infinity                                                   

In the .[.following.]. .Iadd.foregoing .Iaddend.Tables I-III the conicconstant K is 1.3258.

                                      TABLE IV                                    __________________________________________________________________________            SURFACE RADIUS                                                                           AXIAL DISTANCE                                             LENS    (mm)       BETWEEN SURFACES (mm)                                                                          N.sub.e                                                                           V.sub.e                               __________________________________________________________________________        S1  93.9194                                                               L1                 33.5000          1.49354                                                                           57.34                                     S2  658.9023                                                                                 3.5151                                                         S3  -548.1886                                                             L2                 9.0000           1.59495                                                                           30.70                                     S4  287.3305                                                                                 Z1                                                             S5  93.2579                                                               L3                 31.0000          1.64128                                                                           55.19                                     S6  -267.6059                                                                                .2000                                                          S7  418.1418                                                              L4                 8.0000           1.59495                                                                           30.70                                     S8  93.2263                                                                                  Z2                                                             S9  158.2390                                                              L5                 18.0000          1.49354                                                                           57.34                                     S10 -1011.3090                                                                               40.8304                                                        S11 -59.1138                                                              L6                 5.0000           1.49354                                                                           57.34                                     S12 -66.5000                                                              L7                 7.0000           1.43900                                                                           55.00                                     S13 plano                                                                 L8                 10.3000          1.53700                                                                           50.00                                     S14 plano                                                                                    -.0004                                                     __________________________________________________________________________    ASPHERICAL SURFACE DATA:                                                      S  D      E      F      G      H      I                                       __________________________________________________________________________    S2 -.23232E-06                                                                           .42953E-11                                                                           .22237E-14                                                                           .28556E-17                                                                          -.10121E-20                                                                           .55278E-25                             S3  .29504E-07                                                                           .85467E-11                                                                           .52630E-15                                                                          -.51414E-18                                                                          -.11801E-21                                                                          -.28899E-26                             S4  .43027E-06                                                                           .29611E-10                                                                           .28426E-14                                                                          -.27848E-17                                                                           .33655E-21                                                                           .24211E-25                             S7 -.16002E-06                                                                           .92877E-10                                                                          -.36095E-13                                                                           .55299E-17                                                                          -.40886E-21                                                                          -.49637E-25                             S8  .12375E-06                                                                           .17993E-09                                                                          -.43680E-13                                                                           .72048E-17                                                                           .38525E-20                                                                          -.90356E-24                             S9  .21084E-06                                                                          -.50871E-10                                                                           .26661E-13                                                                          -.88729E-17                                                                           .10243E-20                                                                           .16196E-25                             S10                                                                               .66547E-07                                                                           .76684E-12                                                                          -.33818E-13                                                                           .34535E-17                                                                           .92518E-21                                                                   -.30429E-24                                    S11                                                                              -.20945E-06                                                                          -.10691E-09                                                                           .16506E-12                                                                           .83919E-18                                                                          -.23434E-19                                                                           .57245E-23                             __________________________________________________________________________

                                      TABLE V                                     __________________________________________________________________________            SURFACE RADIUS                                                                           AXIAL DISTANCE                                             LENS    (mm)       BETWEEN SURFACES (mm)                                                                          N.sub.e                                                                           V.sub.e                               __________________________________________________________________________        S1  79.2080                                                               L1                 26.0000          1.49354                                                                           57.34                                     S2  620.7167                                                                                 6.2518                                                         S3  -187.0779                                                             L2                 8.0000           1.59495                                                                           30.70                                     S4  863.5535                                                                                 Z1                                                             S5  87.2949                                                               L3                 25.5000          1.64128                                                                           55.19                                     S6  -198.4495                                                                                .2000                                                          S7  864.7596                                                              L4                 8.0000           1.59495                                                                           30.70                                     S8  100.0126                                                                                 Z2                                                             S9  164.8389                                                              L5                 14.0000          1.49354                                                                           57.34                                     S10 -543.7059                                                                                43.8086                                                        S11 -53.9053                                                              L6                 4.0000           1.49354                                                                           57.34                                     S12 -61.0000                                                              L7                 8.7000           1.43900                                                                           55.00                                     S13                                                                       L8                 6.5000           1.53900                                                                           59.00                                     S14                                                                                          -.0003                                                     __________________________________________________________________________    ASPHERICAL SURFACE DATA:                                                      S  D      E      F      G      H      I                                       __________________________________________________________________________    S1  .37138E-07                                                                          -.26219E-10                                                                           .23371E-13                                                                          -.40549E-17                                                                          -.57581E-21                                                                          .37161E-24                              S2 -.28674E-06                                                                           .30963E-10                                                                           .14473E-13                                                                           .16963E-16                                                                          -.84215E-20                                                                          .36035E-24                              S3  .42304E-06                                                                           .66225E-10                                                                          -.89951E-14                                                                          -.61368E-17                                                                          -.12988E-20                                                                           14200E-24                              S4  .10031E-05                                                                           .83141E-10                                                                           .28007E-13                                                                          -.20575E-16                                                                           .44543E-20                                                                          .27305E-23                              S7 -.63512E-07                                                                           .15646E-09                                                                          -.87382E-13                                                                           .30696E-16                                                                          -.93757E-20                                                                          .11178E-23                              S8  .31335E-06                                                                           .25228E-09                                                                          -.77793E-13                                                                           .41139E-16                                                                          -.99160E-20                                                                          .16425E-23                              S9  .23544E-06                                                                          -.11740E-09                                                                           .53503E-13                                                                          -.34734E-16                                                                           .26122E-21                                                                          .45851E-23                              S10                                                                               .15690E-06                                                                          -.11252E-09                                                                           .26590E-13                                                                          -.27785E-16                                                                          -.65235E-20                                                                          .71119E-23                              S11                                                                              -.11893E-05                                                                           .58553E-09                                                                           .29907E-13                                                                          -.25989E-16                                                                          -.22814E-19                                                                          .13396E-22                              __________________________________________________________________________

The conic constant K in Tables IV and V is zero.

The following Table VI is set forth, for purposes of example only, toshow the change in power of the high dispersion negative elements andthe change in power due to aspheric surfaces adjacent to or in thevicinity of the clear aperture to contribute to correction of axialchromatic aberration.

In Table VI the quantity K_(A) is the ratio of the power of the elementat the optical axis of the lens to the power of the overall lens. Thequantity K_(C) is the ratio of the power of the lens element at theclear aperture to the power of the overall lens. The power of the lensesof Tables III-IV is at the shortest EFL.

It will be noted that in the examples of Tables II-V the power increasessubstantially from the optical axis denoted as K_(A) to the clearaperture K_(C). In the example of FIG. 1 and Table I, the first elementL1 goes from positive power at the optical axis A to lesser power at theclear aperture, while the second element L2 changes in power to a lesservalue.

The power of a lens at any height from the optical axis, is calculatedby the equation: ##EQU2## where K is the power of the element at theoptical axis or at a given height from the optical axis;

n is the index of refraction;

C₁ is the local curvature of one surface of the lens element at a givenheight;

C₂ is the curvature of the other surface of the lens element at the samegiven height, and

t is the axial thickness of the lens element.

                  TABLE VI                                                        ______________________________________                                                L1   L2          L4      L5                                           ______________________________________                                        Table I   .755   -.562       -.339                                            K.sub.A                                                                       K.sub.C   .667   -.249       -1.636                                           Table II                                                                      K.sub.A   .573   -.466       -.399                                            K.sub.C   2.386  -3.556      -2.827                                           Table III                                                                     K.sub.A   .35    -.615             -.926                                      K.sub.C   .38    -1.488            -2.061                                     Table IV                                                                      K.sub.A   .634   -.437       -.679                                            K.sub.C   2.253  -5.464      -1.833                                           Table V                                                                       K.sub.A   .628   -.442       -.596                                            K.sub.C   1.467  -3.469      -1.503                                           ______________________________________                                    

The correction of axial chromatic aberration through the use of acombination of positive or negative components can be obtained inaccordance with the following relationship. ##EQU3## where K₁ and K₂ arethe powers of the elements L1 and L2 respectively, V₁ and V₂ are thedispersions of elements L1 and L2 respectively, K is the combined powerof elements L1 and L2, and V is the effective Abbe value of the combinedelements.

For complete correction of axial chromatic aberration V is infinity and##EQU4##

The foregoing exemplifies the considerations of the powers anddispersions of cooperating positive and negative elements.

In the examples of Tables II-V, the power of the positive element L1 ofthe first lens unit G1 increases toward the clear aperture, while thenegative power of element L2 increases in absolute value, both increasesdue to the aspheric surfaces thereof.

In the second lens units G2 of all examples only the negative elementchanges in power from the optical axis to the clear aperture due toaspheric surfaces, since the positive elements of lens unit G2 arespherical. However, if desired an aspheric surfaces(s) could be providedon a power element L3 (or L4 in Table III).

In all embodiments the negative element of a positive negativecombination changes in power in such a way from the optical axis to theclear aperture with respect to the power of the preceding positiveelement(s) to contribute to obtaining a favorable correction of axialchromatic aberration in the vicinity of the clear aperture.

As will be seen from FIG. 1 and Table I, this relationship may besatisfied by making element L1 go less positive adjacent the clearaperture and consequently L2 goes less negative. The elements L1 and L2are plastic and may economically have aspheric surface(s) definedthereon to achieve this effect.

In the examples in Tables II-V contribution to axial color correction isachieved primarily by aspheric surfaces on the cooperating highdispersion negative elements L4 which increase in negative power withaperture height, in conjunction with the corresponding power of theproceeding positive element.

In all examples contribution to correction of axial chromatic aberrationis also made by the combination of the power of elements and thefollowing negative element L4 (L5 in Table III) through high dispersionand related power in regard to the proceeding positive power element(s)in the vicinity of the clear apertures.

What is stated for the values of K/V with regard to FIG. 1 and Table Iis equally valid with respect to the other examples, as well as thecooperating positive and negative elements of lens unit G2 of allexamples.

A lens as shown in FIG. 3 will provide an image quality of up to tencycles/mm (MTF) at an image height of almost two hundred seventy inchesat a substantially same throw distance. This lens is designed for frontprojection and gives excellent image quality for data display and largemagnifications. This lens is designed to be focused by movement ofL1-L5, and L6, L7 deferentially for throw distances of approximately sixand one-half feet to twenty-two and one-half feet while maintaining goodimage quality. The EFL of the lens may vary with focusing from 139.4 mmto 140.9 mm.

The focusing travel of the focusing lens units in FIG. 3 is set forth inthe following Table VII.

                  TABLE VII                                                       ______________________________________                                        EFL (mm)       Z1 (mm)  Z2 (mm)                                               ______________________________________                                        140.2          19.81    4.16                                                  140.9          17.44    3.00                                                  139.4          22.71    5.99                                                  ______________________________________                                    

As will be seen from the foregoing data, lens units G1 and G2 move infixed relation while elements L6, L7 move in fixed relation, butdifferentially with respect to lens units G1 and G2.

The lens of FIGS. 1 and 2 are designed for back projection TV sets andmay be variable in focus by positioning lens units G1 and G2 withrespect to lens unit G3 dependent upon the specifications of TVmanufacturers for the size of the viewing screen. The lenses of FIGS. 1and 2 and Tables I and II respectively, are designed for and give goodoptical performance out to five cycles/mm (MTF).

The lenses of FIG. 4 and Tables IV and V are designed for variablefocusing and permit lens units G1 and G2 to differentially move toachieve variable focusing. Table VIII sets forth the movements of thelens units G1 and G2 for various EFL's and magnifications M.

                  TABLE VIII                                                      ______________________________________                                        Table IV                                                                      EFL (mm)  Z1 (mm)       Z2 (mm)  M                                            ______________________________________                                        138.0     26.25         28.88    -.076                                        140.4     29.10         22.56    -.044                                        142.3     31.37         17.93    -.021                                        ______________________________________                                        Table V                                                                       EFL (mm)  F1 (mm)       F2 (mm)  M                                            ______________________________________                                        113.8     18.98         22.09    -.122                                        114.9     20.50         19.04    -.104                                        117.8     24.81         10.79    -.056                                        ______________________________________                                    

The lens of Table IV provides an excellent MTF out to twelve cycles permillimeter and the lens of Table IV provides an excellent MTF of tencycles per millimeter at focus. Both are well balanced in aberrationcorrection, both monochromatic as well as chromatic.

The powers of the lens units as a ratio to the power of the overall lensare set forth in the following Table IX:

                  TABLE IX                                                        ______________________________________                                                K.sub.1 /K.sub.0                                                                    K.sub.2 /K.sub.0                                                                          K.sub.3 /K.sub.0                                                                      T/fo                                        ______________________________________                                        Table I   .185    1.030       -.708 .239                                      Table II  .168    .79         -.543 .089                                      Table III -.198   1.205       -.878 .107                                      Table IV  .267    1.212       -1.145                                                                              .187                                      Table V   .255    .676        -.946 .165                                      ______________________________________                                    

where K₁, K₂, K₃ are the powers of lens units G1, G2 and G3,respectively, and K₀ is the power of the overall lens, f₀ is the focallength of the lens and T is the spacing between lens units G1 and G2.

The first lens unit G1 is of small optical power and may be eitherpositive or negative. The spacing T/fo is less than 0.4.

It may thus be seen that the objects of the invention set forth, as wellas those made apparent from the foregoing description, are efficientlyattained. While preferred embodiments of the invention have been setforth for purposes of disclosure, modification to the disclosedembodiments of the invention, as well as other embodiments thereof, mayoccur to those skilled in the art. Accordingly, the appended claims areintended to cover all embodiments of the invention and modifications tothe disclosed embodiments which do not depart from the spirit and scopeof the invention.

What is claimed is:
 1. A projection lens for a cathode ray tubecomprising a rear lens unit .[.adopted.]. .Iadd.adapted .Iaddend.to beclosely coupled to a cathode ray tube at the object end thereof, saidlens unit having a strongly concave image side surface,a front lens uniton the image side of said lens .[.comprising.]. .Iadd.consisting of.Iaddend.a positive image side element having a dispersion similar tothe dispersion of crown glass followed by a closely spaced negativeelement of high dispersion similar to the dispersion of flint glass, andanother lens unit providing substantially all of the positive opticalpower of said lens intermediate said front and rear lens units, saidanother lens unit including a negative element of high dispersionsimilar to the dispersion of flint glass on the object side thereof. 2.The lens of claim 1 where said another lens unit comprises two positiveelements both having spherical surfaces.
 3. The lens of claim 1 furtherincluding a corrector lens unit of weak optical power having at leastone aspherical surface positioned between said rear lens unit and saidanother lens unit so that the axial marginal rays from the longconjugate side of said lens intersect said corrector lens unit below theclear aperture thereof.
 4. The lens of claim 3 where said front lensunit and said another lens unit move in fixed relation and said rearlens unit together with said corrector lens unit move in fixed relationbut differentially with respect to said front lens unit and said anotherlens unit to vary the focal length of said lens.
 5. The lens of claim 1where the absolute value of the optical power of said negative lenselement of said front lens unit increases from the optical axis to theclear aperture thereof due to at least one aspheric surface thereof. 6.The lens of claim 1 where said lens elements of said front lens unitcontribute to correction of axial chromatic aberration adjacent theclear aperture thereof.
 7. The lens of claim 1 where the elements ofsaid another lens unit contribute to correction of axial chromaticaberration adjacent the clear apertures thereof.
 8. The lens of claim 1where said negative lens element of said front lens unit changes inoptical power with height from the optical axis of the lens tocontribute to correction of axial chromatic aberration in combinationwith the power of the preceding positive element of said front lensgroup.
 9. The lens of claim 1 where said front lens unit and saidanother lens unit move in the same direction but at differential ratesto vary the focal length of said lens.
 10. A projection lens for acathode ray tube comprising from the image end,a first positive lenselement having at least one aspheric surface followed closely by asecond negative lens element of high dispersion having at least oneaspheric surface, a third strongly positive lens element followed by afourth negative lens element having at least one aspheric surface, and afifth negative lens unit having a strongly concave image side surfaceand adapted to be closely coupled to a cathode ray tube.
 11. The lens ofclaim 10 further including another positive lens element closelyadjacent said third lens element on the image side of said third lenselement.
 12. The lens of claim 10 further including a corrector lensunit of weak optical power having at least one aspherical surfacepositioned between said fourth lens element and said fifth negative lensunit so that the axial marginal rays from the long conjugate side ofsaid lens intersect said corrector lens element below the clear aperturethereof.
 13. The lens of claim 10 where the absolute value of theoptical power of said second negative lens element increases from theoptical axis to the clear aperture thereof due to said at least oneaspheric surface thereof.
 14. The lens of claim 10 where said first andsecond lens elements contribute to correction of axial chromaticaberration adjacent the clear apertures thereof.
 15. The lens of claim10 where said third and fourth lens elements contribute to correction ofaxial chromatic aberration adjacent the clear apertures thereof.
 16. Thelens of claim 10 where said negative lens elements change in absoluteoptical power with height from the axis of the lens to the clearaperture thereof due to the aspheric surfaces on each element.
 17. Aprojection lens for a cathode ray tube comprising a first positiveelement having at least one aspheric surface, a second negative lenselement closely spaced to said first element having at least oneaspheric surface and being of stronger absolute optical power adjacentthe clear aperture thereof than at the optical axis, said second elementhaving a high dispersion similar to the dispersion of flint glass, saidfirst lens element having a dispersion similar to the dispersion ofcrown glass, a third strongly positive lens element having a dispersionsimilar to the dispersion of crown glass, a fourth negative lens elementclosely spaced to said third lens element having a dispersion similar tothe dispersion of flint glass, said fourth lens element having at leastone aspheric surface which increases in absolute optical power from theoptical axis to the clear aperture thereof, and a lens unit having astrongly concave image side surface adapted to be closely coupled to acathode ray tube.
 18. The lens of claim 17 further including anotherpositive lens elements on the image side of said third lens element andclosely spaced thereto.
 19. The lens of claim 17 further including acorrector lens element of weak optical power having at least oneaspheric surface positioned between said fourth lens element and saidlens unit such that the axial marginal rays from the long conjugate sideof said lens interact said corrector lens element below the clearaperture thereof.
 20. The lens of claim 17 where said lens first andsecond elements contribute to correction of axial chromatic aberrationadjacent the clear apertures thereof.
 21. The lens of claim 17 wheresaid third and fourth elements contribute to correction of axialchromatic aberration adjacent the clear apertures thereof.
 22. The lensof claim 17 where said second and fourth negative lens elements changein absolute optical power with height from the axis of the lens to theclear aperture thereof due to at least one aspheric surface on eachelement.
 23. A projection lens for a cathode ray tube comprising fromthe long conjugate side a first lens unit of weak power, including apositive element followed by a closely spaced negative element having atleast one aspherical surface and made out of material with higherdispersion than said positive element; a second lens unit providing mostof the positive power of the lens and including at least one positiveelement followed by a closely spaced negative element having higherdispersion than the preceding positive element; and a third lens unit ofnegative power and including a strong negative element in the vicinityof a cathode ray tube.
 24. The lens of claim 23 where said second lensunit comprises two positive elements.
 25. The lens of claim 23 furtherincluding a corrector lens element positioned between said second andsaid third lens units, said corrector element having at least oneaspherical surface.
 26. The lens of claim 23 where the optical power ofthe negative lens element in said first lens unit changes from theoptical axis to the clear aperture thereof.
 27. The lens of claim 23where the optical power of the negative lens element in said second lensunit gradually changes from the optical axis to the clear aperturethereof.
 28. The lens of claim 23 where said positive and negative lenselements of said first and second units contribute to correction ofaxial chromatic aberration adjacent the clear aperture thereof.
 29. Aprojection lens for a cathode ray tube comprising from the longconjugate a first positive lens element having at least one asphericalsurface followed closely by a second negative lens element of highdispersion having at least one aspherical surface, a third stronglypositive lens element followed by a fourth negative lens element of highdispersion having at least one aspherical surface, and a fifth stronglynegative lens component having a strongly concave image side surface.30. The lens of claim 29 further including another positive lens elementclosely adjacent said third lens element.
 31. The lens of claim 29further including a corrector lens element having at least oneaspherical surface positioned between said fourth lens element and saidfifth strongly negative component adjacent to a cathode ray tube so thatthe axial marginal rays from the long conjugate side of said lensintersect said corrector lens element below the clear aperture thereof.32. The lens of claim 29 where said negative lens elements change inpower with height from the axis of the lens to the clear aperturethereof due to at least one aspherical surface on each element.
 33. Thelens of claim 29 where said first and second elements contribute tocorrection of longitudinal chromatic aberration.
 34. The lens of claim29 where said third and fourth elements contribute to correction oflongitudinal chromatic aberration.
 35. A projection lens for a cathoderay tube comprising a first positive element having at least oneaspheric surface, a second negative lens element closely spaced to saidfirst element and having at least one aspheric surface and being ofstronger absolute power adjacent the clear aperture thereof than at theoptical axis, said second element having a high dispersion similar tothe dispersion of flint glass, said first lens element having adispersion similar to the dispersion of crown glass, a third stronglypositive element with a dispersion similar to the dispersion of crownglass, a fourth negative lens element closely spaced to said third lenselement having a dispersion similar to the dispersion of flint glass,said fourth lens element having at least one aspherical surface andbeing of stronger absolute power adjacent the clear aperture thereofthan at the optical axis, and a fifth negative lens having a stronglyconcave surface facing the long conjugate side and adapted to be closelycoupled to a cathode ray tube.
 36. The lens of claim 35 furtherincluding another positive lens element on the long conjugate side ofthe third lens element and closely spaced thereto.
 37. The lens of claim35 further including a corrector lens element having at least oneaspheric surface positioned between said fourth lens element and saidfifth lens.
 38. A projection lens for a cathode ray tube comprising fromthe long conjugate a first unit of a weak power and including at leastone positive element of low dispersion and one negative element of highdispersion and having at least one aspherical surface; a second lensunit providing most of the positive power of the lens and including atleast one positive element of low dispersion and one negative element ofhigh dispersion, a third lens unit of strong negative power adapted tobe closely coupled to a cathode ray tube, said lens being capable ofchanging magnifications by moving said first and said second lens unitsalong the optical axis in a predetermined manner relative to each otherwhile keeping said third unit stationary relative to a cathode ray tube.39. The lens of claim 38 further including a corrector lens unit havingat least one aspherical surface and positioned between said second andsaid third lens units, said corrector lens unit being kept stationarywith respect to said third unit while said first and second lens unitsare moved to change magnification.
 40. The lens of claim 38 where saidnegative elements in said first and second lens units change in powerwith height from the axis of the lens to the clear aperture thereof tocontribute to correction of axial chromatic aberration in combinationwith the power of the preceding element. .Iadd.
 41. A projection lensfor a cathode ray tube comprising from the long conjugate side:(a) afirst lens unit of weak power comprising first and second elements, thefirst element having a lower dispersion than the second element and thesecond element having at least one aspheric surface, said first andsecond elements having positive and negative powers, respectively, atleast in the vicinity of their clear apertures which serve to correctthe lens' axial chromatic aberration; (b) a second lens unit providingmost of the positive power of the lens comprising third and fourthelements, the third element having a lower dispersion than the fourthelement and the fourth element having at least one aspheric surface,said third and fourth elements having positive and negative powers,respectively, at least in the vicinity of their clear apertures whichserve to correct the lens' axial chromatic aberration; and (c) a thirdlens unit of negative power adapted to be closely coupled to a cathoderay tube..Iaddend..Iadd.
 42. A projection lens for a cathode ray tubecomprising a rear lens unit adapted to be closely coupled to a cathoderay tube at the object end thereof, said lens unit having a stronglyconcave image side surface,a front lens unit on the image side of saidlens comprising a positive image side element having a dispersionsimilar to the dispersion of crown glass followed by a closely spacednegative element of high dispersion similar to the dispersion of flintglass, and another lens unit providing substantially all of the positiveoptical power of said lens intermediate said front and rear lens units,said another lens unit including a negative element of high dispersionsimilar to the dispersion of flint glass on the object side thereof,wherein the lens further includes a corrector lens unit of weak opticalpower having at least one aspherical surface positioned between saidrear lens unit and said another lens unit so that the axial marginalrays from the long conjugate side of said lens intersect said correctorlens unit below the clear aperture thereof, and said front lens unit andsaid another lens unit move in fixed relation and said rear lens unittogether with said corrector lens unit moved in fixed relation butdifferentially with respect to said front lens unit and said anotherlens unit to vary the focal length of said lens..Iaddend..Iadd.43. Aprojection lens for a cathode ray tube comprising a rear lens unitadapted to be closely coupled to a cathode ray tube at the object endthereof, said lens unit having a strongly concave image side surface, afront lens unit on the image side of said lens comprising a positiveimage side element having a dispersion similar to the dispersion ofcrown glass followed by a closely spaced negative element of highdispersion similar to the dispersion of flint glass, and another lensunit providing substantially all of the positive optical power of saidlens intermediate said front and rear lens units, said another lens unitincluding a negative element of high dispersion similar to thedispersion of flint glass on the object side thereof, wherein theabsolute value of the optical power of said negative lens element ofsaid front lens unit increases from the optical axis to the clearaperture thereof due to at least one aspheric surfacethereof..Iaddend..Iadd.44. A projection lens for a cathode ray tubecomprising a rear lens unit adapted to be closely coupled to a cathoderay tube at the object end thereof, said lens unit having a stronglyconcave image side surface, a front lens unit on the image side of saidlens comprising a positive image side element having a dispersionsimilar to the dispersion of crown glass followed by a closely spacednegative element of high dispersion similar to the dispersion of flintglass, and another lens unit providing substantially all of the positiveoptical power of said lens intermediate said front and rear lens units,said another lens unit including a negative element of high dispersionsimilar to the dispersion of flint glass on the object side thereof,wherein said negative lens element of said front lens unit changes inoptical power with height from the optical axis of the lens tocontribute to correction of axial chromatic aberration in combinationwith the power of the preceding positive element of said front lensunit..Iaddend..Iadd. . A projection lens for a cathode ray tubecomprising a rear lens unit adapted to be closely coupled to a cathoderay tube at the object end thereof, said lens unit having a stronglyconcave image side surface,a front lens unit on the image side of saidlens comprising a positive image side element having a dispersionsimilar to the dispersion of crown glass followed by a closely spacednegative element of high dispersion similar to the dispersion of flintglass, and another lens unit providing substantially all of the positiveoptical power of said lens intermediate said front and rear lens units,said another lens unit including a negative element of high dispersionsimilar to the dispersion of flint glass on the object side thereof,wherein said front lens unit and said another lens unit move in the samedirection but at differential rates to vary the focal length of saidlens..Iaddend.